Bump actions/checkout from 4 to 6

Bumps [actions/checkout](https://github.com/actions/checkout) from 4 to 6.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v4...v6)

---
updated-dependencies:
- dependency-name: actions/checkout
  dependency-version: '6'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

.github/workflows/htmlproofer.yml

@@ -7,7 +7,7 @@ jobs:
     env:
       HTMLPROOFER_OPTIONS: '--ignore-urls /www.andreas.org/,/www.kbles.ru/,/github.com\/KK2345\/TinyPassword$/,/github.com\/KK2345\/TinyPassword\/blob\/master\/README.md/,/www.oshec.org/,/github.com\/xuio\/Cynteract$/,/github.com\/mattomatto\/craft$/,/triparts.strikingly.com$/,/facebook.com/varioskybean$/'
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - uses: ruby/setup-ruby@v1
         with:
           ruby-version: '3.3'

.github/workflows/validate.yml

@@ -5,7 +5,7 @@ jobs:
     runs-on: ubuntu-latest
     name: Python Validator
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - uses: actions/setup-python@v5
         with:
           python-version: '3.x'